Here is the Archer character Pam Poovey as she appears in seasons 1 through 4:
And here is Pam in season 5:
How and why did she lose so much weight so quickly?
Per this (disapproving!) article:
Pam’s developed a cocaine addiction as a result of a flight she spent wrapped in a body cast made of cocaine (don’t bother asking), and in the most recent episode, she was revealed to have lost a bunch of weight, ostensibly as a result of her new habit.